The aim of this study was to determine whether activation of protein kinase C reinforced or modulated the Ca2+ signal produced in response to antigen on IgE-primed 2H3 cells. When the concentrations of antigen or the Ca2+ ionophore A23187 were such that both elicited the same increase in cytosol Ca2+ concentration (Ca2+)i, antigen but not A23187 induced secretion. A23187 and the phorbol ester 12-o-tetradecanoyl phorbol 13- acetate (TPA) together stimulated histamine release, whereas TPA alone had no effect. Both the Ca2+ signal and activation of protein kinase C appear, therefore, to be obligatory for secretion. In antigen stimulated cells, however, TPA blocked the antigen-induced (Ca2+)i responses and the release of inositol phosphates, but had little effect on histamine release. Thus the possibility exists that a cryptic signal is generated by antigen independently of protein kinase C activation, the (Ca2+)i response, or the release of inositol phosphates. Suppression of the (Ca2+)i signal and the release of inositol phosphates occur with low concentration of TPA (1-20 nM). Further studies suggest that this suppression results from modification of membrane G-protein that allow coupling of IgE receptors to the catalytic unit, phospholipase C.